1. Field of the Invention
The present invention relates to: a method for manufacturing a domain-inverted region which makes use of an application of an electric field; a wavelength conversion device manufactured by such a method for use in the industrial fields of optical information processing and applied optic measurement control which utilizes a coherent light source and has the domain-inverted region; and a method for manufacturing such a device.
2. Description of the Related Art
The use of a domain inversion phenomenon for forcibly inverting a domain in a ferroelectric crystal makes it possible to form a periodic domain-inverted region (structure) in the ferroelectric crystal substrate. The thus formed domain-inverted region is employed in optical frequency modulators which utilize surface acoustic waves and also in optical wavelength conversion devices utilizing the inversion of a non-linear domain. In particular, by periodically inverting the non-linear domains of non-linear optical materials, an optical wavelength conversion device exhibiting an extremely high conversion efficiency can be manufactured. By using the optical wavelength conversion device thus manufactured to convert a light beam such as that produced by a semiconductor laser, a small-size short wavelength light source which can be applied to various fields such as printing, optical information processing, applied optic measurement control and the like, is realized.
As a conventional method for forming a periodic domain-inverted region, the following methods have been reported: a method utilizing Ti thermal diffusion; a method for adding SiO.sub.2 and then thermally treating it; and a method for performing a proton exchange treatment and a thermal treatment. On the other hand, methods for forming a periodic domain-inverted region, which utilize the inversion of spontaneous polarization of a ferroelectric crystal due to an electric field, have been also reported. For example, a method for irradiating a -C face of a substrate with an electronic beam, and a method for irradiating a +C face of a substrate with positive ions and the like, are employed methods utilizing an electric field. In either case, a deep domain-inverted region having a depth of several hundreds .mu.m is formed by the electric field which is formed by emitted charged particles.
As another conventional method for manufacturing a domain-inverted region, a method for forming a comb-shaped electrode on LiNbO.sub.3 or LiTaO.sub.3 and then applying a pulsed electric field to the comb-shaped electrode has been reported (Japanese Laid-Open Patent Publication Nos. 3-121428 and 4-19719).
With reference to FIG. 30, a conventional method for manufacturing an optical wavelength conversion device is explained.
A conventional optical wavelength conversion device 50 utilizing a LiNbO.sub.3 substrate 55 as shown in FIG. 30 is manufactured as follows. First, a periodic comb-shaped electrode 51 is formed on a +C face 55a of the LiNbO.sub.3 substrate 55, and a planar electrode 52 is formed on a -C face 55b. Then, the +C face 55a is grounded, and a pulse voltage typically having a pulse width of 100 .mu.s is applied onto the -C face 55b by a pulse power source 56. The electric field required to invert a domain is about 20 kV/mm or higher. In application of such an electric field, crystals of the substrate 55 are likely to be destroyed by applying the electric field if the substrate 55 is thick. However, the destruction of crystals due to application of the electric field can be avoided by setting a thickness of the substrate 55 to be about 200 .mu.m, which in turn makes it possible to form a domain-inverted region at room temperature.
Furthermore, a short-periodic domain-inverted structure having a period in the range of 3 to 4 .mu.m is required to realize an optical wavelength conversion device 50 of high efficiency. If the domain-inverted region is formed by applying an electric field, a domain of a portion directly below an electrode is inverted, and then a domain-inverted region spreads out in a direction parallel to the surface of the substrate 55. Therefore, it is difficult to shorten the period of a domain-inverted structure. In order to solve this problem, a short-time pulse having a pulse width of about 100 .mu.s is applied to an electrode so as to shorten a voltage application time period, thereby forming a short-periodic domain inverted structure.
As described above, in the conventional methods, a domain-inverted region can be formed with application of an electric field at room temperature by thinning the substrate 55, and a period of the domain-inverted structure can be shortened by shortening a voltage application time period.
Furthermore, a method for manufacturing an optical wavelength conversion device which utilizes a conventional method for forming a domain-inverted region is disclosed in, for example: M. Yamada, N. Nada, M. Saitoh, and K. Watanabe: "First-order quasi-phase matched LiNbO.sub.3 waveguide periodically poled by applying an external field for efficient blue second-harmonic generation", Appl. Phys. Lett., 62, pp.435-436 (Feb. 1993). In the disclosed method, after periodic domain inverted regions are formed, an optical waveguide is formed so as to perpendicularly cross the periodic domain inverted regions, thereby manufacturing an optical waveguide conversion device. In the manufactured optical waveguide conversion device, a secondary harmonic wave of 20.7 mW is obtained as an output in the case where an interaction length is 3 mm and a power of an incident light beam is 196 mW.
Furthermore, a method for manufacturing a domain-inverted region which employs the combination of a proton exchange and application of an electric field is disclosed in, for example, Japanese Laid-Open Patent Publication No. 4-264534. According to this method, after the entire surface of a substrate is subjected to a proton exchange treatment so as to form a proton-exchanged layer, a comb-shaped electrode is formed on the surface of the proton-exchanged layer and a planar electrode is formed on the bottom face of the substrate. A domain-inverted region is formed by applying a voltage between the electrodes. A proton exchange treatment facilitates the formation of a domain-inverted region. Therefore, it is possible to form a highly uniform periodic domain-inverted structure.
In the conventional methods for manufacturing a domain-inverted region as described above, it is necessary to apply a high (several kV) pulse voltage and a short pulse width (100 .mu.s or less). Since it is difficult to form such a high short-pulse voltage, it is hard to sufficiently ensure reproducibility, reliability and uniformity in application of a voltage.
Moreover, if a high short-pulse voltage is applied to a substrate, electric field distribution becomes ununiform in the substrate planes. Therefore, there arises a problem that uniformity in the planes of the formed domain-inverted structure is deteriorated. Furthermore, since it is difficult to form a uniform domain-inverted structure over a wide area, there also arises a problem that domain-inverted structures cannot be mass-produced utilizing large substrates.
Furthermore, if the applied voltage is ununiform, the substrate can crack, resulting in a decrease in production yield of devices. As described above, in order to prevent the crystals of the substrate from being destroyed even when a high voltage pulse is applied, a thin film substrate can be solely used. However, since it is difficult to handle such a thin film substrate, operability is low.
A short-periodic domain-inverted region is required to realize a highly efficient optical wavelength conversion device. In a conventional method for manufacturing a domain-inverted region which employs application of an electric field, a domain-inverted region spreads out from regions under stripe-shaped electrodes constituting a comb-shaped electrode. As a result, since the adjacent domain-inverted regions come into contact with each other, it becomes difficult to form a short-periodic domain-inverted region.